1. Field of the Invention
This invention relates to a reactive sputtering apparatus, and more particularly belongs to the technical field of thin-film forming processes suited for forming electrodes or protective films for semiconductor elements, electrodes or protective films for liquid-crystal devices, protective films for photomagnetic recording media, reflection preventive films (including anti-reflection coating films) or reflection enhancing films for optical articles, or the like by the use of such an apparatus.
2. Related Background Art
Conventional reactive sputtering is a process in which a mixed gas of a sputter gas and a reactive gas is introduced into a reaction chamber and a metallic target is sputtered to form a metallic compound thin film by a chemical reaction of constituent atoms of the target with the reactive gas. In this instance, the reactive gas may react with the metallic target on the target surface to form a metallic compound on the target surface. Usually, since the sputtering yield with respect to a metallic compound is about 10% of that with respect to the metal, the deposition rate may lower in the reactive sputtering. If the reactive gas is fed at a lower flow rate in order to improve the yield, the metallic compound thin film may be formed as having a higher content of metallic atoms, and can not possibly be a thin film that satisfies the stoichiometric ratio, resulting in poor thin-film characteristics in respect of optical characteristics (such as refractive index and transmittance) and so forth.
Accordingly, some attempts to solve such technical problems are proposed.
FIG. 8 is a schematic view of a reactive sputtering apparatus disclosed in Japanese Patent Application Laid-Open No. 62-56570. Reference numeral 1 denotes a target; 2, a substrate; 3, a supply pipe for argon (Ar) serving as a sputter gas; 4, a supply pipe for oxygen (O.sub.2) serving as a reactive gas; 9, a reaction chamber; 12, a target holder; and 7, a substrate holder.
The above publication states that the use of the apparatus shown in FIG. 8 makes the rate of sputtering larger and brings about an improvement in the characteristics of the oxides, since the sputter gas and the reactive gas are separately introduced, and since the sputtering takes place preferentially in the vicinity of the target and the oxidation reaction takes place preferentially in the vicinity of the substrate.
In reality, however, the sputter gas and the reactive gas mix at a zone between the target and the substrate to form a mixed plasma of both. Especially when a thin film is formed on a large-area substrate, the discharging region between the substrate and the target is so large that the sputter gas and the reactive gas can be separately present with difficulty. Thus, the film quality and the rate of sputtering can not be so much improved as expected.
Meanwhile, FIG. 9 is a schematic view of a reactive sputtering apparatus disclosed in Japanese Patent Application Laid-Open No. 6-41733. Reference numeral 1 denotes a target; 2, a substrate; 3, a supply pipe for argon (Ar) serving as a sputter gas; 4, a supply pipe for oxygen (O.sub.2) serving as a reactive gas; 7, a substrate holder; 8, a power source; 9, a reaction chamber; 12, a target holder; 13, a differential pressure plate; 14, a high frequency power source; 15, a coil; 16, a magnet; 17, a pipe for circulating a refrigerant; and 18, an exhaust pump.
In this apparatus, an exhaust vent communicating with a vacuum pump is provided at an upper part of the reaction chamber 9, and the differential pressure plate 13 is utilized to produce a difference in pressure between the reaction chamber upper part and the reaction chamber lower part so that the sputter gas and the reactive gas can be separated.
Japanese Patent Application Laid-Open No. 7-335553 discloses a reactive sputtering apparatus proposed in order to achieve an object different from the object of the above apparatuses. This apparatus is provided with a collimator between the target and the substrate in order to fill up contact holes of semiconductor devices.
The specification and drawings of U.S. Pat. No. 5,415,753 and a publication "THE SECOND INTERNATIONAL SYMPOSIUM ON SPUTTERING & PLASMA PROCESSES, 1993, pp. 269-274" also disclose a reactive sputtering apparatus provided with a perforated plate between a target and a substrate and so constructed that a sputter gas and a reactive gas are separately fed.
In the apparatus shown in FIG. 9, however, an opening 13a of the differential pressure plate 13 is larger in size than the substrate 2, so that, in reality, the sputter gas will undesirably pass through the opening 13a of the differential pressure plate 13 to flow to the substrate 2 side. Hence, also in such an apparatus, the rate of sputtering and the film quality can not be so much improved as expected.
The apparatus disclosed in U.S. Pat. No. 5,415,753 has also no sufficient countermeasure for gas exhaustion, and the rate of sputtering and the film quality can not be so much improved as expected.